Measurement of a fluid flow-rate is an important practice in a wide variety of applications. For example, in biotechnology or semiconductor applications, the flow rate of gases is critical to growth (of cells, solid-state layers, etc.). Too much flow or too little flow will “spoil” the process result. Flow-measurement can come via several different technologies, such as differential-pressure, thermal-mass-flow, and coriolis-mass-flow, but many others exist as well.
However, not only is the measurement of these flows important, but the end-user is frequently extremely concerned about the control of those flow rates. This control is most typically performed via a “modulating” or “proportional” control-valve, passing more or less fluid (gas or liquid) from a source (e.g. tank) to a process/destination. In other words, as the process needs more or less flow, a “setpoint” (desired flow) will be calculated and communicated to a “loop-controller,” then compared to the actual flow (process-measured-variable), and the difference will cause the motion of the valve (more-open, or more-closed).
As with flow measurement technologies, there are many techniques to build a control-valve, but they typically involve opening or closing a path for fluid to pass through. Typical constructions are: poppet, ball, needle, pinch, guillotine, etc. Each has its performance advantages and disadvantages, as well as cost differences. So, the unique process-application will determine the best choice.
For safety reasons, valves come in several configurations, so that when power is removed from the system, the valve will go open, closed, or will “fail-in-place.” Most often, a normally-closed valve is selected (e.g. stop adding fluid to a process), but a normally-open valve is sometimes specified as well.
In the context of low flow measurement and control devices, the fluid quantities passed through the devices are on the order of several cm3/minute of gas, or several grains/hour of liquids. These small quantities are typically used for a critical component, such as dopants, catalaysts, additives, etc. These low flow devices are also typically quite small. Thus, the manufacture of these small devices is quite demanding. Additionally, the opportunity for a process to block these small devices and associated cavities is high.
In the low flow context, many of the valve construction types mentioned above are simply impractical to build due to small geometries. For manual adjustments, a needle-valve is sometimes employed. The needle valve typically includes a tapered pin that is inserted into or withdrawn from an orifice. As the pin is withdrawn, more orifice cross-sectional area is available for flow to pass. The typical length of the needle is on the order of a centimeter or more. However, as manual controls give way to automated controls, this long tapered needle requires a large positioner to be moved, which causes the valve to become bulky.
Another common low flow valve is a poppet-valve. In a poppet-valve, a flat seat is placed over an orifice to close off flow or lifted a small amount, so that the fluid may flow through an annular ring and then pass through the orifice. Unlike the aforementioned needle-valve, the movement of the poppet, from open to close or vice-versa, represents a very short stroke. Consequently, a solenoid is often the positioner employed with the low flow poppet valves.